1. Field of the Invention
The present invention relates to an ink jet printing apparatus and a printing method of applying ink as a printing agent to a printing medium to form an image.
2. Description of the Related Art
In these days, OA (Office Automation) equipment such as personal computers and word processors have come into wide use and various printing apparatuses have been provided to print information outputted from this equipment on different types of printing media. In particular, ink jet printing apparatuses have various advantageous points such as low noise, low running cost, small structure, relatively easy realization of color printing and the like, and thereby, such printing apparatuses have been used by a wide range of users. Particularly, in recent years, there has been a growing demand for outputting an image taken by a digital camera with the quality equivalent to that of silver-salt photographs. In response to such demand, printing methods to which various devices are applied have been put into practice. In addition, for instance, there have been commercialized and widespread apparatuses capable of printing without margins (hereinafter referred to as a marginless print) on edge portions, such as front and a rear end portions, of the printing medium. However, regarding the front and rear end portions of the printing medium, conveyance accuracy of the printing medium tends to be deteriorated due to the structures of the printing apparatuses. In order to solve such a problem, there have been already provided ink jet printing apparatuses in which a special printing method is adopted for printing on the front and rear end portions of the printing medium. A specific structure of “printing on the front and rear end portions” will be briefly described, below.
(Regarding Printing on Front and Rear End Portions)
Generally, there is a tendency that an image is distorted when the image is printed on the front or the rear end portion of the printing medium. This problem is caused mainly by a state in which the printing medium is separated from a part of plural rollers conveying the printing medium with supporting front and rear portions the printing medium. The following will specifically explain the aforementioned state by using the drawings.
FIG. 41 is a view schematically showing a printing head, a printing medium, and a conveyance mechanism which conveys the printing medium when the central portion of the printing medium is printed. In FIG. 41, a conveying roller M3060 is positioned upstream in a conveying direction, and two paper discharging rollers M3100 and M3110 are positioned downstream. The conveying roller M3060 and the paper discharging rollers M3100 and M3110 work respectively in cooperation with a pinch roller M3070 and two spurs M3120 and M3120. In other words, three nip portions are formed, and a printing medium P is held in the nip portions to be conveyed.
Reference numeral H1000 indicates a printing head cartridge including a printing head in which a plurality of printing elements for ejecting ink are arranged at a predetermined pitch in the direction parallel to the conveying direction shown in FIG. 41.
The printing head cartridge H1000 ejects ink from each of the printing elements while the printing head cartridge H1000 is moving for scan in the direction perpendicular to the figure. Thereby, an image is formed on a region of the printing medium P positioned between the conveying roller M3060 and the paper discharging rollers M3100. The print scan by the printing head cartridge H1000 and the conveyance operation of the printing medium P by three pairs of rollers are alternatively repeated, so that an image is sequentially formed on the printing medium P.
FIG. 42 is a view illustrating a state in which a print operation goes forward from the state shown in FIG. 41 and a print is made on a portion in the vicinity of the rear end portion of the printing medium P. The printing medium P is already separated from the conveying roller M3060 and is conveyed by rotation of only the paper discharging rollers M3100 and M3110.
Generally, in many cases, there are slight differences in a roller diameter and conveyance accuracy between the conveying roller M3060 and the paper discharging rollers M3100 and M3110, because of differences in main functions between these rollers. The main function of the conveying roller M3060 is to position the printing medium at an appropriate position relative to the printing head for every print scan. Accordingly, the conveying roller M3060 has a roller having a diameter large enough to perform the conveyance operation with desired accuracy. In contrast to this, the main function of the paper discharging rollers M3100 and M3110 is to discharge, without failure, the printing medium on which the print has been made. Accordingly, in many cases, their roller diameters are smaller than that of the conveying roller M3060 and their conveyance accuracy of the printing media is inferior to that of the conveying roller M3060. Therefore, the state in which the conveyance accuracy is deteriorated occurs in a region printed between the time when the rear end portion of the printing medium P moves from the conveying roller M3060 and the time when the print on the rearmost end thereof is finished, as compared with the previously printed region. Under this state, if the conveyance amount is insufficient, a so-called “black line” sometimes appears by overlapping, with each other, adjacent image forming portions printed by two successive print scans. Conversely, if the conveyance amount is too large, a so-called “white line” appears by making a distance between the adjacent image forming portions printed by the two successive print scans. Depending on an image to be printed, this causes a non-negligible adverse effect upon the image.
Moreover, an adverse effect is also caused on the image when both end portions of the printing medium are not held. When the rear end portion of the printing medium P moves from the conveying roller M3060, the distance between the printing head and the printing medium (hereinafter referred to as head-to-paper distance) is varied to no small extent, and thereafter becomes unstable. The printing head cartridge H1000 performs the print scans while ejecting ink at timing corresponding to a predetermined head-to-paper distance which is maintained by the front and rear rollers. Then, ink ejected at appropriate timing forms dots on the printing medium and the dots are arranged at a suitable pitch to thereby form an image. Accordingly, if the head-to-paper distance varies during printing, or if a variation in the head-to-paper distance within a print swath is large, positions of the dots on the printing medium become unstable. For this reason, the adverse effect such as generation of the white line, the black line, or a granular impression on an image occurs on the image.
The aforementioned problem of the head-to-paper distance occurs when the print is made on the front end portion of the printing medium as in the case of making the print on the rear end portion thereof.
FIG. 43 is a view illustrating a state in which a print is made on a portion in the vicinity of the front end portion of the printing medium P. Under this state, the printing medium P is held only by the conveying roller M3060 and pinch roller M3070, which are positioned upstream, and thus conveyed. In other words, in the case where the print is made on the front end portion, the paper discharging rollers M3100 and M3110 are not involved in the conveyance of printing medium P. It can be said that the printing medium is conveyed with higher accuracy as compared with the state in which the print is made on the portion in the vicinity of the rear end portion illustrated in FIG. 42. However, the problem of the head-to-paper distance caused by the state in which the front end portion of the printing medium P is not held occurs as in the state shown in FIG. 42. Namely, the position accuracy of dots on the printing medium becomes unstable as compared with that of the print on the central portion of the printing medium (state in FIG. 41), whereby causing deterioration in image quality such as generation of the white line, the black line or the granular impression.
The following measures are taken against the aforementioned adverse effects on the image, which occur at the time of printing on the front and rear end portions of a printing medium, particularly in a serial-type printing apparatus designed in placing importance on the image quality (for example, Japanese Patent Application Laid-Open No. 2002-103584). Specifically, this is a method in which a print swath (that is, a range of the printing elements assigned to be involved for printing in an array of the printing elements actually ejecting the ink) of a printing head is reduced and in which the amount of conveying a printing medium is reduced in conjunction with the reduction in the print swath, when the print is made on the front and rear end portions. The print swath of the printing head is reduced, thereby making it possible to suppress variations in the head-to-paper distance within the print swath. In this case, particularly, execution of multi-pass print to be described later exerts an effect to suppress generation of the granular impression. Even when the conveyance accuracy is deteriorated, conveyance errors can be reduced by decreasing the amount of conveying the printing medium. Moreover, in combination with reduction in the pitch between portions where adjacent image forming portions printed by two successive print scans are connected, the method also exerts an effect to make the white line and the black line less noticeable. In addition, if the multi-pass print is combined, the effect can be more enhanced and the effect to suppress the generation of the granular impression can be also expected. This is because, in the multi-pass print, an image is formed by performing the print scans of multiple times on the same area of the printing medium.
Furthermore, the similar measures are taken in an ink jet printing apparatus which adopts an interlace printing method. In the interlace printing method, by using a printing head in a form having lower density of arrays of printing elements than density of printing, printing an image is completed by performing print scans of multiple times to interpolate the density of printing in the vertical scan direction. That is, the number of printing elements which actually eject ink is reduced only in printing on the front and rear end portions, and in conjunction with this reduction, the amount of conveying the printing medium is adjusted.
However, the inventors of the present invention recognized that it is difficult to suppress both the reduction of image quality and the reduction of printing speed where a reduction ratio of the print swath is uniformly set for printing on the front and rear end portions of the printing medium, regardless of conditions selected for printing. Concrete examples thereof will be described below.
Here, the conditions selected for printing include a condition related to an image print mode and a condition related to a printing medium to be used.
In recent years, with a widespread use of digital cameras, there is also a need for printing with the image quality comparable to that of silver-salt photographs by means of an ink jet printing apparatus capable of outputting a shot image onto a printing medium, such as paper or the like, easily in home-use environment. To that end, recently, there is a case in which a print is made by using six colors of ink including low color density ink such as light cyan ink and light magenta ink in addition to the ink of conventionally used four colors of cyan, magenta, yellow, and black, in order to attempt enhancement of the image quality in print results of images of color toned photographs.
Recently, digital cameras of single-lens reflex type digital cameras are marketed at relatively low prices, and ink jet printing apparatuses are therefore used for printing images of monochrome toned photographs as well as the images of color toned photographs. In the print of an image of the monochrome toned photograph, cyan ink (or magenta ink) and yellow ink are used for correcting color tone, in addition to black ink that serves as a basic tone of the monochrome image. Moreover, in order to lessen a granular impression in halftone, it is put into practice to add gray tone thereto by using light cyan and yellow inks. Namely, in the case of printing the image of the monochrome toned photograph, enhancement of the image quality thereof is attempted by making multi-color printing with a plurality of chromatic colors in addition to black as an achromatic color.
An attempt has been also made to improve the image quality by mounting, on the apparatus, a plurality of inks of achromatic colors (gray ink or the like) with different densities, instead of these multiple chromatic colors, and by making a print using the plurality of inks of achromatic colors with different densities (see Japanese Patent Application Laid-Open No. 2000-177150. Moreover, products are known in which such a print can be made (Photosmart 7960 manufactured by Hewlett Packard, and PM-4000PX manufactured by Seiko Epson).
By contrast, in some cases, all gradations covering from a highlight portion to a maximum print density portion are printed by using ink only of a single color (for example, black ink in a monochrome photo tone image) which can output the maximum print density of the color of a basic tone. In such cases, particularly in a halftone portion, the deviation in dot-landing positions is noticeable. For example, contrast in a monochrome image print is higher than that in a color image print, since, in the monochrome image print, dots of black ink are printed on a white print medium. A portion where dots are locally concentrated due to the deviation of the dot-landing positions tends to become noticeable due to appearance of the black line or the like.
The deviation in the landing positions of dots is caused by variations in nozzle configurations generated in manufacturing processing of ink jet print heads and noise components such as vibration of the apparatus during printing.
As symbolized by the case where an image is printed by using ink of a single color, the deviation of dot-landing positions tends to be more noticeable in the halftone portion as the number of colors of ink used becomes less. The reason will be described as follows.
In general, there is a tendency that the larger number of colors of ink is used, the larger amount of the ink in total is applied to a predetermined area on the printing medium. This results in an increase in ink coverage on the surface of the printing medium. Conversely, there is a tendency that the smaller number of colors of ink is used, the less amount of the ink in total is applied to the predetermined area on the printing medium. This results in a decrease in ink coverage thereon. The deviation of dot-landing positions in higher ink coverage does not affect so much the image quality. If the deviation of dot-landing positions occurs in low ink coverage, however, the image quality is appreciably affected. This is because of the following reason. Specifically, in the case of the low ink coverage, the color of the printing medium itself is more likely to be seen in a larger portion, as compared with the case of the high ink coverage. Accordingly, the deviation of dot-landing positions cyclically varies appearance of the color of the printing medium itself.
Moreover, in the case of the monochrome images, the deviation in dot landing positions becomes even more noticeable because of a higher contrast between the color of black ink and the color of the print medium, in addition to the originally low ink coverage caused by printing with only the black ink. In addition, the conveyance accuracy of the printing medium is deteriorated in printing on the front and rear end portions of the printing medium, as mentioned above. As a result, the adverse effects such as generation of the white line, the black line, or granular impression on the image are outstandingly noticeable. This problem cannot be solved by uniformly reducing the print swath of the printing head for printing on the front and rear end portions.
Furthermore, in actual, properties of the printing media have an influence on the degree of the conveyance accuracy and that of variations in the head-to-paper distance when the printing medium is supported and conveyed only by a roller positioned either upstream or downstream in the conveying direction. Depending on the properties, each of the printing media have differences, from another printing medium, in thickness of the printing media, stiffness thereof, roughness thereof, presence or absence of an ink absorbing layer, the properties of the ink absorbing layer and the like. Accordingly, depending on the type of the printing medium, a coefficient of friction between the printing medium and the roller as well as degrees of curvature caused in the front and rear end portions of the printing medium are different from those of another printing medium.
For this reason, in the case where the print swath and the amount of conveying the printing medium are uniformly set for printing on the front and rear end portions of the printing medium, the following phenomena may occur. In some case, deterioration in the image quality, which may occur in printing on printing the front and rear end portions of a certain type of printing medium, can be effectively suppressed by use of the above-mentioned method. In another case, however, this method may not be effective for another type of printing medium.
Furthermore, when the print swath of the printing head is narrowed to reduce the amount of conveying the printing medium, a time required for making a print on one printing medium is increased accordingly. In the conventional controlling method, since the print swath and the amount of conveying the printing medium are uniformly set for printing the front and rear end portions of the printing medium, a user request cannot be satisfied in some cases. For example, there may be a case in which the user desires to make a print with priority placed on printing quality even if some amount of time is required, while there may be a case in which the user desires to make a print with priority placed on printing speed.
As described in the foregoing, where a reduction ratio of the print swath is uniformly set for printing on the front and rear end portions of the printing medium, regardless of conditions selected for printing, it is difficult to suppress both the reduction of image quality and the reduction of printing speed. Moreover, there is a case in which a user request cannot be satisfied.